Jim Crawford (PI)
Jennifer Olson (Co-I)
NASA Langley Research Center
Mail Stop 483
Hampton, VA 23681
(757) 864-7231
Reflecting the insights gained from analysis of the PEM-West and
PEM-Tropics A data sets, three basic questions will be investigated during
PEM-Tropics B as related to ozone and atmospheric oxidants:
The approach to be taken in the proposed research will be similar to that previously used to analyze the PEM-West and PEM-Tropics A data. A photochemical box model capable of producing both photostationary state and time dependent results will be employed. This model has been recently revised, but is still basically very similar to models previously described by Davis et al. [1993, 1996] and Crawford et al. [1996, 1997b,c]. Typical constraining species/parameters include: O3, CO, NO, H2O, NMHC=s, H2O2, CH3OOH, HNO3, PAN and the physical parameters temperature, pressure, and the UV solar flux.
PSS model output will be most useful in the comparison of model predicted and observed values. This exercise constitutes the single most important method for gaining insight into possible elements of Aincompleteness@ in the model=s chemical mechanism. For PEM-Tropics B, our goal will be to compare with observed values of OH, peroxy radicals, H2O2, CH3OOH, CH2O, and NO2. In particular, this effort will focus on several of the sorties to be flown on the P-3B aircraft while based at Christmas Island. As noted in the NRA two separate sequences of sunrise and sunset flights will be carried out from this tropical environment thus permitting near 24 hour coverage of the diel profiles of the family of HOx related species listed above as well as numerous sulfur species whose concentration levels are critically tied to OH. This will represent one of the most extensive tests of photochemical mechanisms to date.
The time-dependent (TD) mode of the model will be used to provide diel profiles as well as diurnal averages for all transient radical species as well as output products for the photochemical budget of O3 (i.e., F(O3), D(O3), and P(O3)). This model will also be used to evaluate the photochemical budget of NOx. Analysis of NOx sources is important both because of its controlling influence on O3 photochemistry and because of its impact on the partitioning of HOx radicals. This is a particularly important issue in the upper free troposphere since this is a major photochemical source region for O3. The purpose of model calculations as related to the NOx budget will be to determine what fraction of the NOx might be explicable via the recycling of HNO3 and PAN. Previous studies have shown this value to generally be less than about 30% [Davis et al., 1996; Jacob et al., 1996; Crawford et al., 1997b,c]. The assumption then is that the remaining portion of NOx is due to some additional unknown recycling of NOy species or is contributed from primary sources.
A semi-quantitative evaluation of primary sources will be carried out through a statistical analysis of the measured NO and calculated NOx observations. Primary source injections are both episodic and often elevate NOx concentrations well above levels in surrounding air. By contrast, recycling is a relatively slow process serving to sustain background NOx levels. Thus large, short-duration enhancements in observed NO can often be attributed to primary source injections of NOx. As previously shown by Crawford [1997a], statistical removal of these primary source enhancements can lead to estimates of primary source NOx contributions to the total pool of observed NOx. Detailed and comprehensive evaluation of correlations between NO and all other available parameters during these periods of enhancement can often lead to an identification of the source type (e.g., lightning, aircraft, convection of surface emissions, stratospheric intrusion). These methods have been applied previously to PEM-West data [Davis et al., 1996; and Crawford, 1997a] and will also be used in the evaluation of the PEM-Tropics data.
References
Crawford, J. H., et al., Photostationary state analysis of the NO2-NO system based on airborne observations from the western and central North Pacific, J. Geophys. Res., 101, 2053-2072, 1996.
Crawford, J. H., An analysis of the photochemical environment over the western, North Pacific based on aircraft observations, Dissertation,1997a.
Crawford, J. H., et al., Implications of large scale shifts in tropospheric NOx levels in the remote tropical Pacific, J. Geophys. Res., in press, 1997b.
Crawford, J. H., et al., An assessment of ozone photochemistry in the extratropical western north Pacific: Impact of continental outflow during the late winter/earlier spring, J. Geophys. Res., in press, 1997c.
Davis D. D., G. Chen, W. Chameides, J. Bradshaw, S. Sandholm, M. Rodgers, J. Schendal, S. Madronich, G. Sachse, G. Gregory, B. Anderson, J. Barrick, M. Shipham, J. Collins, L. Wade, and D. Blake, A Photostationary state analysis of the NO2-NO system based on airborne observations from the subtropical/tropical North and South Atlantic, J. Geophys. Res., 98, 23501-23523, 1993.
Davis, D. D., J. Crawford, G. Chen, W. Chameides, S. Liu, J. Bradshaw, S. Sandholm, G. Sachse, G. Gregory, B. Anderson, J. Barrick, A. Bachmeier, J. Collins, E. Browell, D. Blake, S. Rowland, Y. Kondo, H. Singh, R. Talbot, B. Heikes, J. Merrill, J. Rodriguez, and R. E. Newell, Assessment of the ozone photochemistry tendency in the western North Pacific as inferred from PEM-West A observations during the fall of 1991, J. Geophys. Res., 101, 2111-2134, 1996.
Jacob, D. J., B. G. Heikes, S.-M. Fan, J. A. Logan, D. L. Mauzerall, J. D. Bradshaw, H. B. Singh, G. L. Gregory, R. W. Talbot, D. R. Blake, and G. W. Sachse, The origin of ozone and NOx in the tropical troposphere: A photochemical analysis of aircraft observations over the south Atlantic basin, J. Geophys. Res.101, 24,235-24,250, 1996.